1. Field of the Invention
The present invention relates to a rotation detector using a magnetoresistance element, and more particularly to such a detector suitable for an engine control system for detecting the rotational angle and speed of an internal combustion engine.
2. Description of Related Art
The timing for igniting fuel in an internal combustion engine (spark timing) is usually determined based on information of a crank angle and a cam angle. In a four-cycle engine, for example, information for determining a cylinder to be ignited is obtained from one rotation of the cam, and information for determining the spark timing is obtained from a crank rotation, because the cam makes one rotation while the crank makes two rotations.
An example of a rotation detector of this kind is disclosed in JP-A-3-195970. A bias magnet is disposed at a vicinity of the outer periphery of a cam gear having gear teeth and gear bases formed alternately thereon. The bias magnet generates a magnetic field directing to the cam gear. A magnetoresistance element (referred to as MRE hereafter) is disposed in the magnetic field between the cam gear and the bias magnet. The MRE detects the rotation of the cam gear from magnetic field alternation caused by rotation of the cam gear.
The rotation detector disclosed in this publication cannot detect whether the MRE faces a tooth of the cam gear or a gear base (a space between two neighboring teeth), when the cam gear is not yet rotated by the engine after the detector is switched on. In other words, the angular position of the cam gear cannot be detected during a short period before the engine starts. The reason for this will be explained, referring to FIGS. 11A and 11B. FIG. 11A shows a situation where a gear base of cam gear 101 faces MRE 103, and FIG. 11B shows another situation where a tooth of cam gear 101 faces MRE 103. Bias go magnet 102 generates a magnetic field directing to cam gear 101 having teeth 101a and gear bases alternately formed thereon. MRE 103 is placed in the magnetic field between cam gear 101 and bias magnet 102. Gear tooth 101a has side surfaces 101b at both sides thereof. The direction of the magnetic field is shown by arrows. Since the magnetic field direction is substantially the same in both situations as shown in FIGS. 11A and 11B, MRE 103 cannot detect the cam gear angle when cam gear 101 is not rotating. Once cam gear 101 rotates, MRE 103 generates signals shown in FIG. 12, in which an upper graph shows the signal when cam gear 101 begins to rotates from the position shown in FIG. 11A, and a lower graph shows the signal when cam gear 101 begins to rotate from the position shown in FIG. 11B. In the upper graph, MRE 103 generates signal U when it meets a first side surface 101b, and signal V when it meets with the next side surface 101b. In the lower graph, signal V is first generated and then signal U is generated. This means that the signals generated in period T from switching-on of the detector to a time MRE 103 first meets side surface 101b are the same in both situations shown in FIGS. 11A and 11B. Therefore, the cam gear angle cannot be detected in period T. Accordingly, a cylinder to be first ignited cannot be determined, and the first cylinder is not fired at a very beginning of the engine start.